The use of pressure sensors for minimally invasive procedures requires increasingly smaller sensors. For example, a pressure sensor instrumented guidewire (Pressure Guidewire) for the assessment of the coronary fractional flow reserve (FFR) is highly demanding as it requires the smallest pressure sensor, while delivering high fidelity pressure measurements.
In the past few years, there has been an increasing number of fiber optic pressure sensors based on the use of a Fabry-Perot cavity. Fabry-Perot sensor can be made of a small diameter and can be made at a low cost as they can be produced using micromachining techniques (Microelectromechanical Systems=MEMS). It is herein worth noting that Fabry-Perot based pressure sensors are quite similar to capacitance based pressure sensors, where pressure measurement is accomplished by measuring the deflection of the diaphragm.
Fabry-Perot based pressure sensors are therefore considered as those having the best potential for numerous applications, and among others the best to suit the needs for catheter and guidewire tip pressure measurement. Numerous methods and designs were proposed for pressure sensors such as those described in U.S. Pat. Nos. 4,678,904 and 7,689,071.
As the size of prior art pressure sensor designs shrinks, Fabry-Perot or others, the sensitivity also diminishes, to a point where adequate resolution, stability and therefore accuracy, are no longer satisfactory.
It is indeed well known by those skilled in the art that as the size of a pressure sensor diaphragm is reduced, the deflection of the diaphragm relative to pressure is reduced as well. One can compensate for such a reduction of the diaphragm deflection relative to pressure by thinning such diaphragm. But this strategy has limitations as discussed below.
FIG. 1 shows a prior art construction of a Fabry-Perot sensor 1 for measuring pressure. A bi-directional fiber optic 2 guides the light signal toward a Fabry-Perot pressure chip (not numbered). The pressure chip is made of a glass substrate 4. One first partially reflective mirror 5 is deposited within a recessed cavity 3 performed on the top surface of the glass substrate 4. A diaphragm 7 is bonded or welded to the glass substrate 4, the internal surface of diaphragm 7 serving as a second mirror 6. Both mirrors 5, 6, spaced apart by a distance given by the depth of the recessed cavity 3, constitutes a Fabry-Perot cavity. The second mirror 6 bows toward first mirror 5 as function of an applied pressure, therefore changing the FP cavity length. The FP cavity length is an unambiguous function of pressure.
FIG. 2 shows the shape of a typical diaphragm 7 deformed as a result of applied pressure. As pressure increases, incremental deflection of diaphragm declines, i.e., the deflection of the diaphragm is non-linear function of the applied pressure. FIG. 3 shows a typical response of same pressure sensor having different diaphragm thicknesses. One can notice that as diaphragm thickness diminishes (from Si-No etch to Si-etch 4), although the sensitivity increases sharply when operating in lowest pressure range, i.e., around vacuum, the sensitivity saturates when operating in higher pressure range, around atmospheric pressure in this case. The increase of sensitivity of an absolute pressure sensor operating with a bias pressure is limited.
In addition to the above sensitivity limitation, the internal stress within the diaphragm increases as thickness of the diaphragm is reduced, potentially leading to diaphragm failure. Risk of diaphragm failure is obviously accentuated by a situation where the system operates with a bias pressure, such as atmospheric pressure. For medical applications that involve catheter tip pressure sensing, the pressure of interest is centered at atmospheric pressure (typically 760 mmHg). Reducing the thickness of a diaphragm increases the sensitivity around 0 mmHga, but increasing the sensitivity around 760 mmHga remains limited.
As a consequence of the above, one major drawback of current Fabry-Perot sensors as they are miniaturized, and similarly of current capacitance based pressure sensor designs, is their lack of adequate sensitivity to pressure. Accuracy, resolution and reliability then often become unsatisfactory, while other undesirable parasitic effects such as moisture drift and thermal effects appear to be amplified relative to pressure.
Accordingly, there is a need for a sensor design having an improved sensitivity for miniaturized sensors.